What is the recommended clearance between cylinder and piston when installing aluminum pistons?

By Dave on Sunday, February 10, 2008 - 01:36 pm:

l believe it should be 0.002" measured from 90 degrees from the pin( sides ), for aluminium, but your pistons should have all of those rebuild tollerences and front direction on a data sheets which should also give you the exact specs of the individual pistons in the box. Better wait for others to confirm this , l think the old cast pistons had a clearance of 0.005"

By aaron on Sunday, February 10, 2008 - 01:58 pm:

With .002" clearance on aluminum pistons that big you will get engine lock-up as soon as it warms up. I'll let those who know through out a number. I know what I think, but I'll keep quiet because I want to know what others think it should be.

I would be carefull and mindfull to get that clearance right before hand.I had a bad experiance with mine ,someone else had bored it and put the pistons in,and they gaulded in less than 15 minutes.Took alot of work with emery paper and a buffer to take some mess off the piston skirts and then hone out the cylinders. It would be wise to listen to the folkes that have built some engines right.I am purty sure there are some rebuilders on the forumn that can give you a good number.I would just as soon it be a bit on the big side as to gauld.

By Glen Chaffin on Monday, February 11, 2008 - 02:03 am:

The same old questions keep coming up time after time. Listen to Richard. I can guarantee that if you use less than 0.0045" with aluminum pistons your engine will seize.

Dumb question time. So if the recommended clearance is .005' for aluminium pistons, does that mean that the bore's diameter should be 0.010" bigger than the piston's overall diameter. Thanks for your patience with my question. David.

FYI... The last set of aluminum pistons I got from one of the vendors had a bright sticker on the lid of the box saying the piston clearance should be .003 -.0035. I looked for the box but it 's long gone. Why would the Mfr. put that on the box if it's not correct?

It may be correct. Aluminum pistons should be able to run with those clearances, provided there is enough skirt taper. The pistons that I've seen scuffed when fitted that tight are scuffed high up the skirt, under the bottom ring lands. The pistons you have may be profiled differently than those that scuff at those clearances.

IMO, if there is sufficient skirt taper and sufficient eccentricity (skirt ovality), aluminum pistons in 3.75" bore size could be set at 0.002" clearance.

Be on the safe side,give it room to grow! I learned from this experiance any engine I have to do machine work to as in boreing,will be sleeved,put back stock and cast pistons will be used.Run 10 more minutes,this would have ruined my engine.

I bet it varies with piston manufacturer. I've been told one manufacturer uses old pots and pans or whatever scrap aluminum alloy is available. I won't mention any names, but it's a four letter word.

Safe clearance also may vary with the design of the piston. The one good source for piston info I've seen is the "Audel's New Automobile Guide" from 1938 to about 1947. It has maybe 75 pages on pistons and rings, including both iron and aluminum pistons.

Before installing new pistons, inspect them carefully. I found one - after failure - with slag in an expansion slot, which prevented expansion.

Also, be sure the pin is beefed up in the area where it is clamped. Clamping will distort an unreinforced pin, believe it or not. After that, it gets ugly. rdr

I think when I posted about this problem orignaly you meantioned due to it being along time since my engine was built there was a company makeing junk pistons. What urks me is everything under the sun has aluminum pistons but for a model T they seem to be a bear to get right.

Any aluminum alloy with more than six percent silicon should be fine for a 3.75" diameter piston in an engine that makes less than 10 horsepower per cylinder and runs no more than 2500 rpm.

The scuff on Mack's piston above is due to inadequate skirt taper which has to be compensated for by excessive clearance at the bottom of the skirt. What a shame to have to start out with "worn out" clearances to prevent this from happening.

Full skirt pistons are no longer in vogue, but when they were 0.001" of clearance per inch of bore was considered acceptable practice - in engines that made far more than 10 horses per cylinder and ran at least twice as fast as the T engine.

Slipper skirt pistons in today's engines are fitted with more like 0.0005" clearance per inch of bore and often make more than 50 horses per cylinder.

What happens with the shape & the lubrication of the cylinders in a side valve engine, with the hot exhaust valve near the bore, compared to the perhaps more uniform cylinder expansion in a OHV engine?

Maybe not a significant difference, just wondering if some of the bigger piston clearence is needed just because of the side valve design?

Interesting question, Roger, especially if the spark were retarded a bit. However, I experienced an almost identical event to the pic above, on #4 in the Fronty when we first got it over 10 years ago. I merely sanded the piston and it's still powering the car.

This engine was put together by the late Fred Upshaw, who was semi-famous for using Chevy crankshafts in T engines. I later found the first 3 rods were from a '27, and #4 from a '28, which was 2 ounces heavier. He made up for half of that with an unreinforced wristpin, which bent under the clamping pressure. I eventually replaced the '28 rod with another '27 rod, and used a proper pin.

Floating pins, a la Model A, don't have to be reinforced through the center.

IMO, yes, the side-valve design is harder on the piston for the reason you state. So also is thermobarf coolant circulation and not using enough spark advance with today's high octane fuels in a low-compression engine like the T.

1 thing for sure though,when Briggs went overhead valve,they didnt leave enough metal between the valves and the cylinder at the top,I cant count how many 14.5 hp engines I have put head gaskets on because of that booboo.

By larry on Sunday, February 17, 2008 - 08:56 am:

In the book i have from the MTFCA engine restoring book states that aluminum pistons need no less that-.006 minimum clearence

I've done about 25 T engines and set the piston clearance at .0035" with no problems. I can tell you from experience that if you set them at .006" to .008" they will be noisy (piston slap that you might not notice right away, but after a few hundred miles you sure will!). The .006 to .008 clearance that a lot of people come up with is information from old books that is outdated and incorrect for the currently available aluminum pistons. If you just tell your machinist to fit the pistons, they will likely do it to modern standards which is way to tight (likely .002") and they are very likely to sieze! Just out of curiosity, we called several vendors a couple years ago and asked them what clearance to use and got answers that varied from .0025" to .008, so don't rely on your vendor for the "best" clearance! By the way, the pistons that come in the plain white boxes don't seem to be as good as the ones that come in the red, white, and blue boxes, but the vendors claim they are from the same manufacturer. We have also encountered wrist pin issues with the currently available aluminum pistons, so make sure your wrist pins fit properly. If they don't, then send them back for another set! We haven't had any wrist pin to piston failures, but have found plenty of wrist pins that fit too tight (the bores are okay, but it is as if the bores are not aligned).